We
demonstrate a high mobility, triple-stack ZnO/AlZnO/YZnO heterojunction
thin-film transistor (TFT) using the semiconductors deposited by spray
pyrolysis at 350 °C on an Al2O3 gate insulator.
A thin layer (5 nm) of AlZnO on the top of ZnO used as an active layer
of an inverted coplanar-structured TFT increases the field-effect
mobility (μFE) from 42.56 to 82.7 cm2 V–1 s–1. An additional 5 nm thick YZnO
on the top of the ZnO/AlZnO TFT improves the electrical stability
by reducing the defects in the bulk ZnO, AlZnO, and at the interface
AlO
x
/ZnO. The ZnO-based materials show
a nanocrystalline structure with the grain size less than 20 nm. The
triple-stack oxide TFT shows a μFE of 71.3 cm2 V–1 s–1 with a threshold
voltage (V
TH) of 2.85 V. The hysteresis
voltage for pristine ZnO, ZnO/AlZnO, and ZnO/AlZnO/YZnO TFTs is 0.52,
0.24, and 0.02 V, respectively. The ZnO/AlZnO/YZnO TFT shows a negligible V
TH shift under temperature bias stress for 3600
s at 60 °C and excellent environmental stability over a few months,
which is due to the presence of stronger Y–O and Al–O
bonds in the back channel. The threshold voltage shift under positive
bias temperature stress for pristine ZnO, ZnO/AlZnO, and ZnO/AlZnO/YZnO
TFTs is 0.78, 0.40, and 0.15 V, respectively. Compared to the pristine
ZnO TFT, the ZnO/AlZnO/YZnO TFT shows better environmental and bias
stabilities with improved hysteresis. The experimental data of ZnO/AlZnO
and ZnO/AlZnO/YZnO TFTs can be fitted by technology computer-aided
design (TCAD) simulation using the density of states model of the
oxide semiconductors. From the TCAD simulation, it is found that a
2D-like electron gas is formed at the narrow AlZnO layer between ZnO
and YZnO.